This invention relates generally to the testing of aircraft fuel for contaminants, and more particularly to a test unit which separates the fuel from the contaminant whereby the presence of the latter is positively indicated.
As noted in the Advisory Circular (20-43B), published June 8, 1971 by the Federal Aviation Administration of the Department of Transportation, the existence of any contaminant in aircraft fuel is dangerous. Fuel is said to be contaminated when it contains substances that are not called for in the fuel specification. In general, these substances are water, rust, sand and other foreign matter.
All aviation fuel absorb moisture from the air and include water in both dissolved and liquid form. The amount of dissolved water contained in the fuel depends on the fuel temperature. Whenever the temperature is decreased some of the dissolved water comes out of solution and slowly falls to the bottom of the fuel tank. With an increase in fuel temperature, water is then drawn from the atmosphere to maintain a saturated solution. Thus changes in fuel temperature give rise to a continuous accumulation of water. When the fuel system in a plane is subjected to freezing temperatures, this water may turn to ice and restrict or arrest the flow of fuel.
Rust in the fuel is produced in pipelines, storage tanks and in fuel trucks which handle the fuel. Because such rust is found in fine particle sizes, a high degree of filtration is necessary to remove the rust constituent from the fuel. As to dust and sand, these contaminants enter through openings in the tank or result from the use of dirty fuel-handling equipment.
As a practical matter, the total elimination of contaminants from aviation fuel is not possible. Thus one effective way of preventing water contamination is to fill the fuel tank completely at the end of each flight, thereby avoiding condensation of water on the walls of a partially-filled tank. But this post-flight procedure is usually not feasible under prevailing conditions. Hence the present practice is to carefully test for contaminants before flight.
Preflight testing, as presently carried out, involves draining a generous sample of fuel into a transparent container from each of the fuel sumps and from the main fuel strainer or gascolator. The fuel samples are then examined for dirt and water. If contaminants are present, they will collect at the bottom of the container. Since the fuel has a distinctive color, the difference between the layer of fuel in the container and the water and dirt therebelow should be readily evident. When contamination is detected in a given sump, fuel is repeatedly drained therefrom and tested in the container until the system appears to be clear of all water and dirt.
The difficulty with the existing preflight procedure to test for fuel contaminants is that it is not entirely effective under poor conditions of illumination when it is difficult to discern the difference in color between the fuel dye and the water. Moreover, if the water contains rust or other material imparting some color thereto, the distinction between the color of the water and the color of the fuel may become less pronounced.
Another factor that must be taken into account is that while fuels are color-coded in accordance with their octane content, should fuels be mixed, as is sometimes the case, the resultant color may approach a neutral value. Thus 80/87 octane has a red dye added thereto, 91/96 octane has a blue dye added thereto, 100/130 octane is green and 115/145 octane is purple. But the mixture of any two of these fuels may result in a much less distinctive color whose value may be difficult to distinguish from dirty water.
Hence the existing procedure in which the contaminants and the fuel mingle in the same container and are distinguishable from each other only in terms of color contrast, is lacking in reliability and does not afford an unmistakable, positive indication of the presence of contaminants.